ABSTRACT In Peru, HIV disproportionately affects men who have sex with men (MSM) and transgender women (TGW), who have an estimated prevalence more than 20 times that of the general population. Treatment as prevention (TasP) is a biomedical method of preventing HIV using early initiation of antiretroviral therapy (ART) to reduce the level of HIV in bodily fluids, thus reducing the likelihood of HIV transmission. Pre-exposure prophylaxis (PrEP) is a prevention method that involves giving antiretroviral medication to HIV-negative people to prevent acquisition of HIV. While these methods have been shown to be highly effective in preventing HIV among MSM and TGW, they do not protect against other sexually transmitted infections (STIs), and there are concerns that these HIV prevention strategies will lead to risk compensation ? changes in behavior resulting from changes in perceived level of personal risk. In the context of HIV prevention, risk compensation could take the form of decreases in condom use or other changes in sexual behavior that may increase STI risk. The primary goal of this proposal is to measure changes in sexual behavior and STI rates among Peruvian MSM and TGW who are using TasP and PrEP. To do this, we plan to first develop a contextual framework describing overall trends of STI prevalence and patterns of sexual behavior among Peruvian MSM and TGW during the past two decades, using data from a series of cross-sectional surveys conducted in this subpopulation in Peru between 1996 and 2011. Then we will test whether ART use is associated with changes in sexual behavior or STI incidence in a cohort of MSM and TGW participating in a TasP trial, using linear regression and two-sample z- tests to describe overall patterns in sexual behavior over time, and differences in sexual behavior by ART status. Finally, we will test whether initiation of PrEP is associated with changes in STI incidence or sexual behavior in a cohort of MSM and TGW utilizing PrEP, using longitudinal data and mixed effects regression models to describe changes in magnitude and direction of STI incidence and sexual behavior over time, identifying both immediate and delayed changes in behavior and STI risk. This study will provide valuable information on whether sexual behavior changes as a result of using antiretroviral therapy to prevent HIV, and when these changes are taking place. This information can be used to develop new strategies to address potential increases in STI risk associated with these new HIV prevention methods, and to identify when these strategies would be best applied.